Various industrial fabrication processes often employ apparatus for dispensing pre-selected, discrete amounts or doses of a fluid medium for anyone of many different purposes. For example, in connection with semiconductor fabrication operations for producing semiconductor devices from silicon wafers, various processing equipment produces waste water of which the pH value must be controlled. This is usually accomplished by measuring the pH of the waste water and then adding doses of a buffer solution to the waste water from time to time such that the pH value of the waste water is maintained within a pre-selected, allowed range.
For instance, waste water which is produced by an ion implantation equipment usually contains a concentration of fluorine ions. Based on the regulations of environmental protection agency, such concentration can not be higher than 15 ppm when the waste water is released into the environment. It is therefore important that the concentration of fluorine ions in the waste water must be determined accurately. When determining the fluorine ion concentration, it was found that the fluorine ion concentration varies with the pH values of the waste water except in a relatively narrow range. For instance, FIG. 1 shows the electrical potentials for aqueous solutions that contain different concentrations of fluorine ions plotted against the pH values of the aqueous solutions. As shown in FIG. 1, three different fluorine ion concentrations, i.e., 2 mg/l, 5 mg/l and 10 mg/l plotted. For all three concentrations, the electrical potential is not constant at various pH levels except within a relatively narrow range of between about 4 and about 10.
The measurement of fluorine ion concentrations in waste water therefore presents a challenge in that the pH value of the waste water must first be adjusted to within the range of pH where the electrical potentials are constant so that the fluorine ion concentration can be reliably determined.
Conventionally, a buffer solution which is substantially of alkaline nature is added to the aqueous solution to stabilize the chemistry prior to a pH determination. For instance, a 1% alkaline solution of CH.sub.3 COONa has been utilized as the buffer solution. The buffer solution is normally added to the waste water at a constant rate regardless whether it is necessary or whether the waste water is of an acidic nature. The conventional apparatus for dispensing such buffer solutions is therefore inadequate in providing a stabilized aqueous solution for the detection of fluorine ion concentrations. This is particularly the case when a waste water is already alkaline and the addition of the buffer solution further increases its alkalinity.
It is therefore an object of the present invention to provide an apparatus for determining concentration of fluorine ions in an aqueous solution that does not have the drawbacks or shortcomings of the conventional apparatus.
It is another object of the present invention to provide an apparatus for determining concentration of fluorine ions in an aqueous solution that is capable of producing accurate results.
It is a further object of the present invention to provide an apparatus for determining concentration of fluorine ions in an aqueous solution that is capable of making pH adjustment in the solution prior to the determination of the fluorine ion concentration.
It is another further object of the present invention to provide an apparatus for determining concentration of fluorine ions in an aqueous solution which includes a pH controller, an acid dispenser, a base dispenser and a fluorine ion sensor.
It is still another object of the present invention to provide an apparatus for determining concentration of fluorine ions in an aqueous solution which is capable of adjusting automatically a pH value of the solution to within a 4.about.10 range before determining the concentration of fluorine ions.
It is yet another object of the present invention to provide an apparatus for determining concentration of fluorine ions in an aqueous solution which incorporates an acid dispenser that dispenses H.sub.2 SO.sub.4 and a base dispenser which dispenses NaOH.
It is still another further object of the present invention to provide a method for determining concentration of fluorine ions in an aqueous solution by first adding H.sub.2 SO.sub.4 or NaOH to the solution such that the pH value of the solution falls within the range of 4.about.10.
It is yet another further object of the present invention to provide a method for determining concentration of fluorine ions in an aqueous solution by adding a buffer solution, an acid or a base into the aqueous solution until a pH value between 4 and 10 is obtained.